The invention is particularly applicable to centrifugal pumps having an internal liner although reference to this particular application is not to be taken as a limitation to the scope of the invention. It will be readily apparent to those persons skilled in the art that the invention is also applicable to pumps which do not have an internal lining.
FIG. 1 is a schematic sectional side elevation of part of a typical centrifugal slurry pump currently in use. The pump generally indicated at 10 comprises an elastomeric liner 11 which is mounted within a rigid housing (not shown). The liner 11 includes a main liner 12 and a front liner 13 (often referred to as a throat bush). The main liner may be formed of two parts. Such is well known in the art and it is not proposed to discuss it in detail here.
The pump 10 further includes an impeller 15 comprising a front shroud 16 and rear shroud 17. A series of passageways 18 are formed between the shrouds these passages being separated from one another by blades or vanes 19. The pump 10 has an inlet 20 and each passage has a passageway inlet 21 and a passageway outlet 22. The pump inlet 20 is shown as having a diameter D.sub.1, the passageway inlet 21 is shown as having a width B.sub.1 and the passageway outlet is shown as having a width B.sub.2. The outer diameter of the impeller is shown as D.sub.2.
All centrifugal pumps have a flowrate at which their efficiency is at a maximum. This is called the Best Efficiency Point (BEP) flowrate.
FIG. 2 is a graph for a typical centrifugal pump plotting the head (or pressure) of the pump against flow rate. The BEP flowrate is that when the graph reaches its highest point.
At lower or higher flows, the efficiency is less than at the BEP point. The BEP flowrate is determined by the pumps geometry. The most practical and cost effective method of producing pumps is to design pumps with a fixed geometry to suit a particular duty. Normally the pumps BEP flowrate will be made to coincide as close as possible to the required or duty flowrate in order to achieve the most economical operation.
Once a pump's geometry is fixed, then the BEP flowrate can only be changed to a small degree. The design and manufacture of variable geometry centrifugal slurry pumps is not economical. Changing the internal liner shape of the configuration of the impeller is possible in order to make small changes to the BEP flowrate. However, such changes are expensive as patterns and molds require alteration to change the geometry. This particularly applies to the pump liners.
In some instances, the required or duty flowrate specified by a customer is higher than the BEP flowrate for the available fixed geometry pumps. In this case the efficiency will be lower than optimum and would result in higher running costs. This situation might arise if the duty flowrate is higher than the largest pump available, or the duty flowrate fell between two fixed pump models. In both cases it is logical to increase the BEP flowrate of the smaller pump if the increase required is in the order of up to 35% higher.
The BEP flowrate is determined amongst other parameters by the width of the pump liners and the impeller. To increase the BEP flowrate, the impeller needs to be made wider. As it is not practical or economical to change the main pump liners, the outlet width of the impeller cannot be increased.
Typically it will only be the flowrate that needs to be increased and the head (pressure) and speed of the pump would remain approximately the same. Increasing only the pump flowrate, increases a pumps specific speed. This is a non-dimensional number incorporating the pump flowrate, head and speed and is universally applied to characterize a pump's design. The specific speed and hence the pump head can be improved by changing the design of the impeller.
Typically in currently known centrifugal pumps the widths of the passageway inlet and outlet are approximately the same. Furthermore the inclination angle .beta. as shown in FIG. 1 is in the range from 0 to 15.degree..
The inclination angle is defined as the angle between a line joining the mid points of the passageway inlet and outlet widths to a line at right angles through the passageway outlet width.